Multi-stage refrigeration processes are typically classified as either a mixed refrigerant cycle or a cascade refrigeration cycle. In the mixed refrigerant cycle, a refrigerant of specialized composition is employed to chill the fluid from ambient conditions to a state where it can be liquefied using an expansion valve.
In the typical cascade refrigeration cycle, successive expansion valves are used to gradually liquefy the fluid. The partially liquefied fluid is then distributed to a flash drum. The liquid from the flash drum is distributed for further chilling to subsequent flash drum stages. Vapors from the flash drums are compressed and condensed with a refrigerant.
In FIG. 1, a schematic diagram illustrates a conventional cascade refrigeration system 100 for ethylene export. An ethylene feed stream 101 at supercritical conditions from a pipeline is dehydrated using a two-bed dehydration unit. The dehydration unit operates in batch operation, where one bed 102 is dehydrating the ethylene feed stream 101 and the other bed 103 is regenerating. In regeneration mode, a portion of the dehydrated ethylene stream 111 from dehydration bed 102 enters a regeneration heater 104. The heated dehydrated ethylene stream 111 then enters dehydration bed 103 to regenerate dehydration bed 103. A water saturated ethylene stream 105 from dehydration bed 103 is condensed in an air cooler 106 and removed using a knock-out drum 107, which is also referred to as a separator, to separate the water saturated ethylene stream 105 and a condensed water stream 108. The water saturated ethylene stream 105 is compressed in a compressor 109 and the compressed water saturated ethylene stream 110 is returned to mix with ethylene feed stream 101.
The remaining portion of dehydrated ethylene stream 111 is chilled through three separate heat exchangers 112, 113, 114. Each heat exchanger cools the dehydrated ethylene stream 111 using a conventional propylene refrigerant system shown with dotted lines. The chilled dehydrated ethylene stream 115 is let-down to its condensation pressure at ambient conditions using let down valve 117 to produce flashed ethylene stream 118. The flashed ethylene stream 118 enters a flash drum 120, which is also referred to as an economizer, where it is mixed with a recycled ethylene stream 135 and flashed. The flashed ethylene vapor stream 122 mixes with a lower pressure compressed ethylene stream 124, which is then compressed in a compressor 125 to produce a higher pressure vapor ethylene stream 126. The vapor ethylene stream 126 is subsequently chilled through the propylene refrigerant system using three separate heat exchangers 128, 130, 132. The chilled condensed liquid ethylene stream 133 enters an accumulator 134 where any inert substances are vented in the accumulator 134 as they build up in the process and the recycled ethylene stream 135 is produced.
A liquid ethylene stream 136 from the flash drum 120 is expanded through an expansion valve 138 to produce a chilled two-phase fluid ethylene stream 140. The chilled two-phase fluid ethylene stream 140 enters another flash drum 142 where it is flashed. The flashed vapor ethylene stream 144 is mixed with a compressed ethylene stream 157 and then compressed in a compressor 145 to produce the compressed ethylene stream 124. The compressed ethylene stream 124 is then mixed with the higher pressure flashed ethylene vapor stream 122. The liquid ethylene stream 146 from flash drum 142 is expanded through another expansion valve 148 to produce a chilled two-phase fluid ethylene stream 150. The chilled two-phase fluid ethylene stream 150 enters another flash drum 152 where it is flashed. The flashed vapor ethylene stream 154 is mixed with a compressed ethylene boil-off-gas stream 163 and then compressed in a compressor 155 to produce the compressed ethylene stream 157. The liquid ethylene stream 156 is either distributed to a cryogenic tank 158 for storage or transported to another site. The ethylene boil-off-gas stream 160 from the cryogenic tank 158 is compressed in a compressor 162 to produce the compressed ethylene boil-off-gas stream 163.
While a cascade refrigeration cycle is the easiest to operate because of its reliance on a single refrigerant, it can be less energy efficient than a mixed refrigerant process. This is because a cascade refrigeration system employs staged flashes to primarily recover energy, whereas a mixed refrigerant system can be closely matched to the cooling curve of the commodity to be chilled. Traditionally, energy recovery involving the expansion valves in both processes has focused on hydraulic expanders or turbines, which add complexity and capital cost because they require mechanical equipment, hydraulic seals and a sink to utilize the recovered energy. The recovered energy is thus, not typically redeployed in the process itself. Liquid motive eductors have also been employed in refrigeration processes, but have either been used as a replacement for refrigerant compression or as a means to control the liquid refrigerant level, rather than taking advantage of the staged flashes present in a cascade refrigerant system to recover energy.